There's a new feature, that "Have you finished watching this title?" is greyed out (i.e. not clickable) for ongoing dramas.
However, for some dramas, this is not the case -- it's still possible to mark a review as "completed" for those.
I tried several random dramas from the front page, and I think marking as "completed" seems to be possible for dramas that have no end date:
not possible to mark a review as complete:
= all have an end date on the drama page
possible to mark a review as "complete":
= don't have an end date
